Recent developments have shown the efficacy and advantages of vertical-cable seismic survey systems. Such systems have proven particularly useful in surveying operations which require a uniform distribution of azimuths and economical three dimensional pre-stack depth migration, as in very deep operations in the North Sea. Improved data quality has allowed a more confident interpretation of low-angle fault scarp features, as well as a more detailed structural model of reservoirs.
However, such systems utilize conventional hydrophone arrays arranged in a vertical pattern. The seismic signals presented to the plurality of hydrophones in the vertically displayed arrays are received and converted to electrical signals in the conventional manner, with allowances for the vertical rather than more standard horizontal orientation of the hydrophone arrays.
Thus, such systems remain limited in the depth for exploration that can be reliably conducted due to the attenuation of useful return signals. These system would be significantly enhanced if some manner of improving the signal/noise ratio could be found. The recognition of this limitation of known vertical-cable seismic systems provided an initial stimulus for the development of the present invention.
A known vertical hydrophone system for acquiring and processing marine seimic data includes cables which are suspended vertically in the water column with an anchor at one end, a buoy at the other, and a recording buoy at the surface. A vessel on the surface fires a pattern of shots, spaced apart by 10 to 25 meters, typically, covering the area around and between the cables. Once one swath of data is completed, the cables are moved to another location, and the process is repeated until the desired area of coverage is acquired.
Very accurate seismic data has been acquired with such a system, but the system presents certain drawbacks. Most apparent is the lost time in laying, retrieving, and relocating the anchored bouy system. Significant time would be saved if the system could acquire data without being anchored to the bottom. Another apparent drawback is the precision required to lay out the anchor system on the bottom in a predicable and repeatable pattern.
The system previously described has apparent advantage of having the vessel separated from a conventional recording streamer. However, this system includes a buoy at the surface for recording seismic data, and is therefore necessarily limited in the amount of data which can be stored at the surface buoy. This data must also be retrieved before another set of shots can be initiated.
Thus, there remains a need for a vertical hydrophone array system and method that is mobile, and does not require periodic relocation to survey additional territory. The system should be capable of directly reporting seismic data to the survey vessel, and should be capable of completing an entire marine seismic survey without periodic scheduled stops.